Stratigraphy and Sedimentary Basins

In a typical depositional environment, changes in geologic conditions through time lead to the deposition of different horizontal layers of rock, each with a unique composition. The sequence of horizontal layers is referred to as the stratigraphic sequence. A younger rock will always be found higher up in a stratigraphic sequence than an older one.

An understanding of stratigraphy is so important to oil and gas exploration that a geologic specialist called a stratigrapher is a key figure in oil and gas exploration.

In the following video, we’ll meet a stratigrapher and learn a bit more about why stratigraphy is so important to the oil and gas industry.

Transcript

Stratigraphy – Hilary Olson – The University of Texas at Austin

I want to talk for a bit about stratigraphy, but in order for it all to make sense, I think we need to start by looking at the interior structure of the earth.

If you were to take a slice out of the Earth, what you would see at the center is a very hot solid core, made up of a mixture of iron and nickel.

When I say hot, I mean that the core is about as hot as the surface of the sun! You would think at those temperatures the core would be liquid, but in fact, only the outer core is – the inner core is under such intense pressure that it is solid.

The next layer up is called the mantle, and although predominantly a solid, the high temperatures make it able to flow very slowly. The very thin veneer at the top of the upper mantle is cool enough to act as a rigid solid, and together with the earth’s crust, it forms a shell around the outside of the planet. This outer shell of rock is a very small percentage of the diameter of the Earth. In relative terms it’s like the skin of an apple.

Now, if you were to zoom in on a slice to get a closer look at the crust, what you would see is that a number of these rocks are made of layers, and these layers are different depending on where you look.

Each of these layers is called a stratum. When you have more than one, they’re called strata. The branch of geology that studies layers of rock is called stratigraphy, and that’s what I do.

Stratigraphers try to understand when all these layers were laid down, under what circumstances and what has happened to them since then.

There’s a lot going on. While they’re being formed, the layers can incorporate all kinds of different things in them, like the remains of plants and animals that eventually become fossils — or different kinds of elements, isotopes and minerals that are clues to answer questions such as: How old are the strata? Were the environment and climate such that we are likely to find a lot of organic material preserved in the layers?

After they’re laid down, the strata can move, they bend, they break, things push up through them, and sometimes they will slide underneath or on top of each other. It can get pretty complicated.

The reason this is relevant to the oil and gas industry is that operators want to know where target formations are – what layer do they drill to and which strata are they going to drill through to get there?

They need to know if there are any hazards on the way down – and now they need to know this in the horizontal direction as well. For example: hazards like faults or salt canopies that they’re going to have to go through that might cause wellbore stability problems or stuck pipe.

So how do stratigraphers figure out what’s there? Well, one way is to start with outcrops. Layers we can see on the surface and that we can follow across great distances.

We can also look at seismic reflection data to see below the ground — this is really important in places where we haven’t drilled yet.

Once we drill, we can add cuttings and wireline logs into the mix to really pinpoint where the different layers start and end and what those layers contain. Using all of these data together, we can make subsurface maps of the geological layers and their structure — both in areas new to oil and gas exploration, as well as better-known regions where we are developing known oil and gas fields.

Understanding the local stratigraphic column is extremely important for everyone’s safety and the economic viability of the well, and because of that, geologists who study stratigraphy are integral to the planning and execution of almost every modern well.

Petroleum geologists need to know where potential reservoirs are located, and what subsurface hazards might be present. The ultimate goal of the stratigrapher is to help provide this information. Since strata are often continuous over large areas, an understanding of the local stratigraphy of an oil or gas play allows geologists to predict what they will encounter before drilling even begins.

Let’s talk a bit more about how strata form.

Sedimentary Basins

As mentioned in the video, sedimentary rocks often form in the presence of water. As water flows downhill, it carries sediments eroded from the land surface. Eventually, these materials are carried to oceans and inland seas. These watery locales are known as sedimentary basins.

In modern and ancient geologic systems, the size of clastic grains delivered to a basin depends on the energy content of the water carrying them. Rapidly moving water can carry large grains, while slow moving water can only carry the finest grains. Since the water emptying from the mouth of a river loses energy as it moves away from the shore of a basin, the grain sizes in each strata decrease with increasing distance from the river’s mouth.

For example, sandstones form near ancient shorelines, while fine grained shales form far from shore. Because of this, a stratum, which has a constant age, may contain more than one type of rock. Lateral regions of a stratum containing a certain rock type are called facies.

Here’s the story so far: sediments are eroded from the land surface and carried by water to a basin. As they move away from shore and the water slows, larger particles drop out of the water column. The rocks that form on the floor of the basin are coarse grained near shore and fine grained further out.

But this leaves one important question: what happens when a basin fills up? We know that sedimentary rocks can reach thicknesses of up to 20 km, greater than the depth of the deepest parts of the ocean. How did these rocks form? Why didn’t the oceans fill completely with sediments?

It all comes down to tectonic forces. Keeping our question in mind, let’s take a look at the role tectonic forces play in geology.

Images: “Goosenecks of the San Juan River Utah” by Robert_Ford via iStock; “Sediment Entering a Lake” by City of Tuscaloosa, Alabama